Erythropoiesis is the production of red blood cells, which occurs to offset cell destruction. Erythropoiesis is a controlled physiological mechanism that enables sufficient red blood cells to be available for proper tissue oxygenation. Naturally occurring human erythropoietin (hEPO) is produced in the kidney and is the humoral plasma factor which stimulates red blood cell production (Carnot, P and Deflandre, C (1906) C. R. Acad. Sci. 143: 432; Erslev, A J (1953 Blood 8: 349; Reissmann, K R (1950) Blood 5: 372; Jacobson, L O, Goldwasser, E, Freid, W and Plzak, L F (1957) Nature 179: 6331–4). Naturally occurring EPO stimulates the division and differentiation of committed erythroid progenitors in the bone marrow and exerts its biological activity by binding to receptors on erythroid precursors (Krantz, B S (1991) Blood 77: 419).
Erythropoietin has been manufactured biosynthetically using recombinant DNA technology (Egrie, J C, Strickland, T W, Lane, J et al. (1986) Immunobiol. 72: 213–224) and is the product of a cloned human EPO gene inserted into and expressed in the ovarian tissue cells of the chinese hamster (CHO cells). The primary structure of the predominant, fully processed form of hEPO is illustrated in SEQ ID NO:1. There are two disulfide bridges between Cys7–Cys161 and Cys29–Cys33. The molecular weight of the polypeptide chain of EPO without the sugar moieties is 18,236 Da. In the intact EPO molecule, approximately 40% of the molecular weight are accounted for by the carbohydrate groups that glycosylate the protein at glycosylation sites on the protein (Sasaki, H, Bothner, B, Dell, A and Fukuda, M (1987) J. Biol. Chem. 262: 12059).
Because human erythropoietin is essential in red blood cell formation, the hormone is useful in the treatment of blood disorders characterized by low or defective red blood cell production. Clinically, EPO is used in the treatment of anemia in chronic renal failure patients (CRF) (Eschbach, J W, Egri, J C, Downing, M R et al. (1987) NEJM 316: 73–78; Eschbach, J W, Abdulhadi, M H, Browne, J K et al. (1989) Ann. Intern. Med. 111: 992; Egrie, J C, Eschbach, J W, McGuire, T, Adamson, S W (1988) Kidney Intl. 33: 262; Lim, V S, Degowin, R L, Zavala, D et al. (1989) Ann. Intern. Med. 110: 108–114) and in AIDS and cancer patients undergoing chemotherapy (Danna, R P, Rudnick, S A, Abels, R I In: M B, Garnick, ed. Erythropoietin in Clinical Applications-An International Perspective. New York, N.Y.: Marcel Dekker; 1990: p. 301–324).
Known pharmaceutical compositions have at least one of the following disadvantages:                they are lyophilisates. Besides the complicated manufacturing procedure, lyophilisates have the disadvantage that they have to be reconstituted before injection into humans. This makes additional handling by medical personnel necessary, which is inconvenient and bears the risk of incorrect handling of the pharmaceutical product;        they contain human serum albumin as an additive. As human serum albumin is a product derived from human body fluid, there is a risk of viral infections by contaminants in the albumin preparation. Also, allergic reactions are possible;        all presently commercially available erythropoietin compositions are unstable at elevated temperatures, i.e. above refrigerator temperature which is usually between 2 and 8° C. Therefore, they have to be stored in a refrigerator (2–8° C.) and cannot be stored at room temperature (around 20° C.). This leads to increased costs, caused by storage and shipment at low temperature and also causes inconvenience in handling of the drug product. Unstable in this context means that storage at elevated temperatures, e.g. 25° C. for a prolonged period of time (i.e. several months, or more than 6 months) leads to degradation of the protein. Degradation in this context describes physical changes (e.g. aggregation or denaturation) and chemical changes (e.g. oxidation or modification of chemical bonds in general) of the protein molecule which are known to occur preferably at elevated temperatures (above 8° C.). Incubating a protein near or above its transition temperature (which is also called melting temperature) leads to unfolding of the protein, i.e. the native structure and the biological activity of the polypeptide is lost. The transition temperature is strongly correlated with the temperature stability of the protein and is dependent on the environment of the protein (e.g. pH, salts, ionic strength, buffer substance, etc.) For example, denaturation may lead to aggregation of erythropoietin molecules, i.e. formation of dimers (covalent or non-covalent), higher order aggregates and even particulates. This leads to reduced potency of the drug and might induce unwanted side effects after injection into humans.        
The problem underlying the present invention is therefore to provide a composition which is able to minimize or suppress the above mentioned disadvantages.